It is conventionally known that as methods of forming a thin film, typically, a sputtering method, an ionized vapor deposition method, etc. are used in the PVD field and a plasma CVD method is employed in the CVD field.
FIG. 1 shows a CVD apparatus using a high-frequency glow discharge of a capacitive coupled type. A high-frequency power system (5), a high-frequency power supply electrode (1), an opposed ground electrode (2), a substrate (3) having a surface on which a thin film is formed, and a plasma region (4) generated between the flat electrodes (1) and (2) installed in parallel are designated. This manner as shown in FIG. 1 is to form a film using a self bias acting in the side of the substrate (3) installed in the side of the high-frequency power supply electrode (1).
Also, FIG. 2 shows an inductive coupled CVD apparatus. In this manner as shown in FIG. 2, a plasma region (4) is formed by applying an induction energy from a coil (6) for high-frequency excitation and ions of a material activated in the plasma region are introduced into a filmy substrate (3) by an electric field from an auxiliary electrode for applying an external bias and a thin film is formed on the flexible substrate (3). Also, this manner is to move the filmy substrate sequentially by a cylindrical roller and a guide roller and form a thin film sequentially on the substrate (3).
When a thin film having a large compressive residual stress, typically such as a thin film of diamond-like carbon, is formed using the CVD apparatus as shown in FIG. 1, force by which the formed film is warped with the surface upwardly curved to a protrusion shape is applied to the thin film since the diamond-like carbon thin film has a large compressive stress of the order of 10.sup.10 dyne/cm.sup.2.
The above condition will be described in FIG. 5. FIG. 5(A) shows the condition in which a thin film (13) having a compressive residual internal stress is formed on a substrate (3). When the compressive residual internal stress acts on the thin film (13), the thin film (13) tends to warp as shown in the drawing. In this case, of course, a stress occurs between the thin film (13) and the substrate (3), and thereby problems such as the decrease in adhesion of the thin film (13) to the substrate (3) or the cracking or peeling of the film (13) occur.
When a flexible and filmy substrate is used as the substrate, particularly, the thin film will curl outwardly and the substrate will curl inwardly.
Also, when the CVD apparatus shown in FIG. 2 is used, the compressive residual stress in the longitudinal direction of the movement of the substrate (3) is canceled by winding the substrate. Though any significant problem does not occur in this case, the substrate will still curl after forming the film thereon due to the compressive residual internal stress in the transverse direction of the substrate (3).
The compressive residual stress in the transverse direction of the substrate (3) cannot easily be restored to its original condition even if corrections are performed later. Even if the compressive residual stress be restored to its original condition, now the stress still remains in the interface between the thin film and the substrate, and cracking, peeling, etc. are induced due to the stress. Taking the long view, therefore, the CVD apparatus in FIG. 2 is lacking in the reliability.
The above problems occur more or less in the thin film formed by the CVD (chemical vapor deposition) method.